Messages in this thread | | | Subject | Re: [PATCH] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg() | From | "Xu, Yanfei" <> | Date | Mon, 5 Jul 2021 23:25:29 +0800 |
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On 7/5/21 11:07 PM, Peter Zijlstra wrote: > [Please note: This e-mail is from an EXTERNAL e-mail address] > > On Mon, Jul 05, 2021 at 04:00:01PM +0200, Peter Zijlstra wrote: > >> No, when try_cmpxchg() fails it will update oldp. This is the reason old >> is now a pointer too. > > Since you're not the first person confused by this, does the below > clarify? > > --- > Subject: Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg() > From: Peter Zijlstra <peterz@infradead.org> > Date: Mon Jul 5 17:00:24 CEST 2021 > > There seems to be a significant amount of confusion around the 'new' > try_cmpxchg(), despite this being more like the C11 > atomic_compare_exchange_*() family. Add a few words of clarification > on how cmpxchg() and try_cmpxchg() relate to one another. > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > Documentation/atomic_t.txt | 41 +++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 41 insertions(+) > > --- a/Documentation/atomic_t.txt > +++ b/Documentation/atomic_t.txt > @@ -271,3 +271,44 @@ because it would not order the W part of > SC *y, t; > > is allowed. > + > + > +CMPXHG vs TRY_CMPXCHG
CMPXHG -> CMPXCHG
> +--------------------- > + > + int atomic_cmpxchg(atomic_t *ptr, int old, int new); > + bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new); > + > +Both provide the same functionality, but try_cmpxchg() can lead to more > +compact code. The functions relate like: > + > + bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new) > + { > + int ret, old = *oldp; > + ret = atomic_cmpxchg(ptr, old, new); > + if (ret != old) > + *oldp = ret; > + return ret == old; > + }
I tried to search some comments about atomic_try_cmpxchg(), but failed. Maybe I missed it. With your this document, it is more clear now.
> + > +and: > + > + int atomic_cmpxchg(atomic_t *ptr, int old, int new) > + { > + (void)atomic_try_cmpxchg(ptr, &old, new); > + return old; > + } > + > +Usage: > + > + old = atomic_read(&v); old = atomic_read(&v); > + for (;;) { do { > + new = func(old); new = func(old); > + tmp = atomic_cmpxchg(&v, old, new); } while (!atomic_try_cmpxchg(&v, &old, new));
Some unnecessary spaces before "while".
Thanks, Yanfei
> + if (tmp == old) > + break; > + old = tmp; > + } > + > +NB. try_cmpxchg() also generates better code on some platforms (notably x86) > +where the function more closely matches the hardware instruction. >
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